Thus, French patent application No. 2 481 755 describes and shows a pressurized fluid (motor or pump) mechanism, e.g. a hydraulic motor having at least two selectable distinct non-zero cylinder capacities, the mechanism being constituted by: at least one first external connector for connection to a source of fluid under pressure, and at least one second external connector for connection to a fluid exhaust; at least one cam whose profile corresponds to a plurality of successive undulations, said undulations being divided at least into first and second distinct groups of undulations with each undulation comprising a first slope and a second slope; a cylinder block mounted to rotate relative to said cam about an axis of rotation and which includes a plurality of cylinders disposed substantially radially relative to said axis of rotation; each cylinder having at least one piston slidably mounted therein and suitable for bearing against the cam and delimiting, within said cylinder, a working chamber which is connected to a communication face of the cylinder block; a plurality of cylinder ducts each connecting one of the working chambers to said communication face of the cylinder block; a fluid distribution face constrained to rotate with the cam, said face permanently bearing against the communication face of the cylinder block and having first and second fluid distribution ducts opening out therein and corresponding to said first and second slopes of each of the undulations, said distribution ducts being suitable for being put into communication, during relative rotation of the cylinder block and the cam, with the cylinder ducts of the various cylinders; and cylinder capacity selection means comprising at least a first cylinder-capacity selection slide valve disposed between said first and second external connectors and the first and/or second distribution ducts corresponding to the undulations of the second group of undulations; said first cylinder capacity selection slide valve having a first position suitable for corresponding: (a) to the first distribution ducts corresponding to the undulations of both groups of undulations being put into communication with the first external connector, and (b) to the second distribution ducts corresponding to the undulations of both groups of undulations being put into communication with the second external connector, and having a second position suitable for corresponding: (a) to the first distribution ducts corresponding to the undulations of the first group of undulations being put into communication with first external connector, (b) to the second distribution ducts corresponding to the undulations of said first group of undulations being put into communication with the second external connector, and (c) to the first distribution ducts corresponding to the undulations of the second group of undulations being permanently isolated from the first external connector and/or to the second distribution ducts corresponding to the undulations of said second group of undulations being permanently isolated from the second external connector.
In such mechanisms where the cylinder capacity is selected by deciding to connect the normal feed system to the cylinders which at each instant are disposed facing the cams either of both groups of cams or of one of the groups only, it is also known to provide the pistons facing respective ones of the groups of cams with different strokes. French patent No. 2 127 268 shows a mechanism of this type.
All of these mechanisms operate well, but their particular design generally makes them complex and expensive to manufacture.
Preferred embodiments of the invention remedy these drawbacks of prior mechanisms by providing a structure which is simpler and suitable for providing the cylinder capacity selections desired by a user, in particular for providing a selection between three different non-zero cylinder capacities and a zero cylinder capacity, without requiring expensive machining.